1. Field of the Invention
The present invention relates to a lead for use with a lithium-ion secondary cell, a lead ribbon, a lithium-ion secondary cell and a method of sealing a container of the lithium-ion secondary cell.
2. Description of the Related Art
Recently, as a demand of cordless and portable electronic devices increases, there have been developed a variety of portable electronic devices which are miniaturized, made light in weight and thin in thickness one after another. Concurrently therewith, a battery serving as an energy source of such an electronic device shares a large ration of the whole of the electronic device. Further, as an electronic device becomes a multifunction electronic device, a power consumption thereof increases so that a capacity of a cell unavoidably increases considerably, thereby resulting in a volume of a secondary cell being increased. Thus, there is an increasing demand of a miniaturized secondary cell having a high energy density.
As secondary cells that have been used heretofore, there are known a lead storage battery and a nickel-cadmium battery. Also, as a new secondary cell, a nickel-hydrogen cell and a lithium-ion cell are now commercially-available on the market. Since these secondary cells use liquid as an electrolyte, they cannot avoid a problem of a leakage thereof. A solidification of electrolyte, i.e. solid electrolyte battery is a powerful means for solving the problem. As typical powerful means, there is a polymer lithium-ion secondary cell using a polymer electrolyte in which a plasticizer is mixed into a polymer. Thus, it becomes possible to manufacture a secondary cell having no risk of leakage and which may be miniaturized, made light in weight and reduced in thickness, thereby resulting in a secondary cell with a high energy density.
As a fundamental arrangement of a polymer lithium-ion secondary cell, a polymer lithium-ion secondary cell is generally comprised of a positive electrode, a negative electrode and a polymer electrolyte. While a variety of polymer electrolytes are developed, electrolytes such as polyacrylonitrile (PAN), polyethylene oxide (PEO), polyvinylidene fluoride (PVdF) and so on are typically known as major electrolytes.
An arrangement of a polymer lithium-ion secondary cell will be described next.
FIGS. 1A and 1B are respectively diagrams showing a structure of a polymer lithium-ion cell obtained when a polyacrylonitrile (PAN) system gel electrolyte is used. As shown in FIGS. 1A and 1B, an activator made of LiCoO.sub.2 and graphite is laminated on a positive electrode current collector 9 made of an aluminum thin plate and an activator made of MCMB, carbon and natural graphite is laminated on a negative electrode current collector 14, which form electrodes. An isolating material (polypropylene, etc.) called a separator is disposed between the positive electrode current collector 9 and the negative electrode current collector 14, and a polyacrylonitrile (PAN) system gel electrolyte is filled into clearances thereof, thereby resulting in a sandwich structure being obtained.
As a container for the sandwich structure, the product is packed/packaged by a laminate material made of an aluminum film and a plastic film. In that case, a cell may be reduced in thickness and increased in capacity by selectively laminating one elemental cell (unit cell) having the sandwich structure as shown in FIG. 2A, rewinding one unit cell as shown in FIG. 2B or folding one unit cell as shown in FIG. 2C or combining the above-mentioned laminated structure, the rewound structure or the folded structure.
Then, the assembly process of the polymer lithium-ion secondary cell will be described with reference to FIGS. 3A to 3D to FIGS. 5A to 5E.
Initially, in the mixing process shown in FIG. 3A, a positive electrode material or a negative electrode material is manufactured by preparing/mixing materials made of an activator, a conductive material, a binder, a volatile solvent or the like. In the next coating process, as shown in FIG. 3B, this positive electrode material or negative electrode material is coated on the positive electrode/negative electrode current collector by a roll coater, baked and then dried. While the roll coater has been described as an example of a coating method, the coating method is not limited thereto, and any method may be used so long as the positive electrode material or the negative electrode material may be coated uniformly. In the next press process as shown in FIG. 3C, the resultant electrode material in which this positive electrode material or the negative electrode material is baked and dried on the positive electrode/negative electrode current collector is pressed in the equal direction by an interlaminar press treatment, thereby resulting in an electrode density being increased. In the next slitter process, as shown in FIG. 3D, the resultant product in which the electrode material is pressed in the equal direction by this interlaminar press treatment is cut as a ribbon-shaped having a constant width.
In the next vacuum dry process shown in FIG. 4A, the resultant product of ribbon-shape having the constant width is dried in the vacuum as shown in FIG. 4A. According to the next lead welding process, in the resultant process which was dried in the vacuum as shown in FIG. 4B, a lead 3 is welded to the surface of a metal on which the positive electrode material or the negative electrode material is not coated. In the next electrolysis solution vacuum impregnation process as shown in FIG. 4C, the electrolysis solution is impregnated into the positive electrode material or the negative electrode material by using vacuum suction. In the next electrolyte gel coating and rewinding process as shown in FIG. 4D, a gel electrolyte is uniformly coated on both surfaces of the separator, and the separator, the positive electrode current collector in which the positive electrode is formed and the negative electrode current collector in which the negative electrode is formed are rewound in the order of the positive electrode current collector, the separator and the negative electrode current collector, thereby resulting in a unit cell being formed. At that time, a unit cell having a width and a laminated thickness matched with a required arbitrary size and a cell capacity may be completed by selecting a unit cell rewinding method, a unit cell laminating method, a unit cell folding method or the like.
In the packing process shown in FIG. 5A, the product in which the separator, the positive electrode current collector and the negative electrode current collector are rewound in the order of the positive electrode current collector, the separator and the negative electrode current collector is packed into a laminate film (e.g. three-layer structure of polyethylene terephthalate/aluminum film/non-elongated polypropyrene) which serves as a thin container 5 for a polymer lithium-ion secondary cell. In the next press process as shown in FIG. 5B, a resultant product in which the unit cell is packed into the container is pressed. In the next vacuum sealing process as shown in FIG. 5C, only the lead is exposed from the container thus pressed with the unit cell under reduced pressure atmosphere, and one side of the container is sealed. Although a heat fusion-bonding method (hot plate adhesion method, impulse adhesion method, ultrasonic adhesion method, high-frequency adhesion method and hot-air adhesion method) is convenient as a method of sealing a laminate film, so long as a sealing performance and a moisture permeability resistance are excellent, an adhesive system and an adhesive coating method (hot-melt method and cold-glue method) are also possible. In the next charging and discharging method as shown in FIG. 5D, it is inspected by repeatedly charging and discharging a resultant product in which the container with the unit cell therein is sealed whether or not a predetermined battery characteristic is obtained. After the above-mentioned processes, there is completed a polymer lithium-ion secondary cell as shown in FIG. 5E.
Here, it is known that various material that are used in the polymer lithium-ion secondary cell are very sensitive to water in the atmosphere and that a barrier property and a moisture permeability resistance of the laminate film obtained after the container was sealed become factors which influence the life span of the battery.
As shown on the table 1 below, these factors are caused by reliability of heat fusion-bonding method used in the heat fusion-bonding portions of upper and lower laminate films, materials of laminate films, shapes of leads and problems such as bonding property/adhesive property/sealing performance of the lead and the laminate film.
TABLE 1 High Ultra- fre- Impulse sonic quency Hot-air Hot plate bonding bonding bonding bonding Plastic film adhesive method method method method polyethylene film .circleincircle. .circleincircle. .largecircle. -- .circleincircle. non-elongated .circleincircle. .circleincircle. .circleincircle. -- .circleincircle. polypropyrene film elongated .largecircle. .circleincircle. .circleincircle. -- .circleincircle. polypropyrene film nutrient -- -- -- -- -- cellophane moisture-proof .circleincircle. .largecircle. .largecircle. -- -- cellophane acetate film .largecircle. .largecircle. .largecircle. .largecircle. .largecircle. hard vinyl .largecircle. .largecircle. .largecircle. .circleincircle. .circleincircle. chloride film soft vinyl .circleincircle. .largecircle. .largecircle. .circleincircle. .circleincircle. chloride film polyvinylidene .largecircle. .largecircle. .largecircle. .circleincircle. .largecircle. chloride film polystyrene film .largecircle. .circleincircle. .circleincircle. -- .largecircle. polyvinyl alcohol .circleincircle. .largecircle. .largecircle. .largecircle. .circleincircle. film polyester film -- .largecircle. .largecircle. -- .largecircle. polycarbonate .largecircle. .circleincircle. .circleincircle. -- .largecircle. film nylon film .circleincircle. .circleincircle. .circleincircle. .largecircle. .largecircle. polyethylene .circleincircle. .circleincircle. -- -- -- cellophane Notes: .circleincircle.: available methods .largecircle.: possible methods --: difficult or extremely-disadvantageous methods
On the other hand, in the manufacturing process of the above-mentioned polymer lithium-ion secondary cell, in the vacuum sealing process, i.e. the process for sealing the unit cell into the container made of laminate film under pressure reduced atmosphere, a sealing work based on the heat fusion-bonding method is frequently used from standpoints of low cost, quality and work property. At that time, the plastic film material which serves as the heat fusion-bonding portions on the upper and lower two innermost layers of the laminate film is limited to limited plastic materials because of an affinity of gel electrolyte and contained solvent. In the case of polyacrylonitrile (PAN) system gel electrolyte, for example, the plastic material is limited to polyolefin (polyethylene, polypropyrene, etc.) which does not contain base.
Also, in the vacuum sealing process, due to unstable heat fusion-bonding conditions and materials of laminate film, there arise the following problems.
Specifically, when conditions of a temperature, a pressure and a time in the heat fusion-bonding are optimum, as shown in FIG. 6A, the heat fusion-bonding is properly effected on heat fusion-bonding portions 2 of sealant layers 19. However, when the conditions of the temperature, the pressure and the time in the heat fusion-bonding are fluctuated to provide an excessive heat fusion-bonding, as shown in FIG. 6B, the sealant layer 19 within the laminate film is broken/removed to expose and heat fusion-bond an aluminum layer 18 (conductor) which is used as a film intermediate layer to improve a moisture permeability resistance. Moreover, when the aluminum film 18 comprising the laminate film is exposed from the cut end face of the laminate film, it is frequently observed that the aluminum film 18 that is exposed from the cut end face of the laminate film contacts with a positive electrode lead 23 and a negative electrode lead 24. This contact becomes a main cause to cause the positive electrode lead 23 and the negative electrode lead 24 to be short-circuited through the aluminum film 18 within the laminate film.
To avoid this drawback, there has hitherto been adopted a method of preventing the aluminum film 18 exposed from the cut face of the laminate film and the lead 3 from contacting with each other by a folded portion 7 provided only at the sealing portion of the electrode as shown in FIG. 7. In this case, the cut face of the laminate film is folded and fixed by a tape.
However, the above-mentioned conventional polymer lithium-ion secondary cell encounters with the following problems.
Specifically, as described above, since the plastic film materials which serve as the heat fusion-bonding portions on the upper and lower two innermost layers of the laminate film are limited to the polyolefin materials such as polyethylene or the like and polypropyrene which contains no base because of an affinity of gel electrolyte and contained solvent in the case of polyacrylonitrile (PAN) system gel electrolyte, for example.
However, these materials have poor adhesive property/bonding property with leads (aluminum, nickel or SUS serving as a positive electrode current collector or copper and the like for a negative electrode current collector), have poor container's moisture permeability resistance, sealing performance, barrier property and safety and have a poor mechanical peel strength of the heat fusion-bonding portion of the container.
Also, as described above, when the conditions of the temperature, the pressure and the time of the heat fusion-bonding are fluctuated to provide the excessive heat fusion-bonding, this becomes the main cause to cause the positive electrode lead and the negative electrode lead to be short-circuited through the aluminum film within the laminate film. As a countermeasure for solving this problem, there is adopted the method of forming the folded portion only on the portion in which the electrode is sealed.
However, although the short-circuit between the positive electrode and the negative electrode through the aluminum film exposed from the end face of the laminate film may be avoided according to this method, it is unavoidable hence productivity/yield/reliability of quality of the polymer lithium-ion secondary battery are lowered and hence a manufacturing cost is increased.
Also, in the heat fusion-bonding process of the container, a heat should be sufficiently conducted from a heating apparatus to the sheath layers of the laminate film upper films and the laminate film lower layer, the aluminum films and the sealant layers and the leads. It takes a long time to effect the heat fusion-bonding by this heating, which causes a productivity to be lowered.
Moreover, since the width of the heat fusion-bonding portion of the container is difficult to be increased due to various conditions, there is a limit in increasing the area in which the heat fusion-bonding portion and the lead are bonded together. As a result, there is then the defect that a sealing performance of a cell container may not be improved much more.
Furthermore, in the above-mentioned lead welding process, leads of slit-shape should be supplied to the production line one by one. There is then the defect that it is difficult to smoothly and automatically supply the leads to production facilities in consideration of the automation.